Support `$ref`-type parameters, as well as components.parameters

[drwpow]

Followup from #236.

We don’t yet support the following schema:

paths:
  '/some/path':
    get:
      parameters:
        - "$ref": "#/components/parameters/since"
        - "$ref": "#/components/parameters/per_page"
        - "$ref": "#/components/parameters/page"

Add support for $ref parameters as well as add the components.parameters object to the final response.

There’s an existing spec which illustrates this: tests/v3/specs/github.yaml.

[gr2m]

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